This invention relates to a focus detecting device in a camera in which the image of an object is divided into two parts by means of a prism, a half mirror, and a mirror. Alternatively, the same image of an object is received at two places in accordance with a so-called range-finder system. By utilizing the fact the amount of mutual displacement of the two parts of the image becomes zero when correct focalization is obtained, the direction in which the photographing lens should be turned or moved for the correct focalization is informed to the photographer by display means such as light emission diodes or the like even if the initial focus position is remote from the correct focalization position.
A number of electrical focus detection methods have been proposed in the art, in which the same image of an object is processed in two optical paths according to variation of spatial frequency or variations in constrast of the image. The two images thus obtained are made to coincide with each other when the correct focalization is obtained. Among these conventional electrical focus detecting methods, the object of most methods resides in obtaining the correct focalization automatically. Accordingly, in these conventional methods, the photographer only knows that the correct focalization has been obtained. Thus, the conventional methods are still unsatisfactory to photographers in many respects.
Heretofore, the focalization can be achieved by moving a photographing lens in presently available camera systems. However, in the case of the aforementioned electrical focus detecting methods, it is necessary to move the photographing lens by means of, for instance, an electric motor. It is therefore a requirement of such systems to provide such an electric motor, a motor drive device and a battery, all of which leads to an increase in size of the camera. Thus, the use of such a camera is necessarily limited and cumbersome.
In addition, there are other factors to consider in the focalization process. At present, there are avilable a variety of optical focus detecting methods in which, for instance, a mat surface, a microprism or a split-prism are utilized. In any of these conventional optical focus detecting methods, the photographer recognizes the image of an object as an analog variation, or a continuous variation to thereby detect the correct focalization. Accordingly, the detection of correct focalization involves personal errors based on user skills. These are due to photographer's personal ability such as visual power, skill in focalization and sense of depth of field. The techniques are therefore not suitable for beginners, or children.
Since electrical focus detecting methods have been proposed in order to overcome this difficulty, completely automatic focalization is not necessarily required. That electrical lens movement is not always required. What is necessary is an indication as to how correct the focalization can be achieved. That is, if display indicating conditions of rear focalization, the front focalization and the correct focalization can be made during operation of the photographing lens as in present camera systems, then any photographer can obtain the correct focalization readily irrespective of his personal ability.